A Magnetic Tunnel Junction (MTJ) sensor is advantageous in terms of high sensitivity, small size, low cost, low power consumption, and the like. The MTJ sensor has high magnetoresistance, and it is compatible with standard semiconductor manufacturing processes; however, the method for preparing a high-performance MTJ linear magnetic field sensor has not yet been fully developed. In particular, it is difficult to effectively control the temperature dependence and hysteresis.
A magnetic field sensor is comprised of individual magnetoresistive elements. During actual application, the magnetoresistive elements are generally connected to form a bridge to eliminate offset, increase the sensitivity, and compensate temperature dependence. The bridge structure can compensate the temperature dependence to some extent; however, the dependence of the intrinsic magnetic performance of the magneto-resistor of the sensor on the temperature will not be completely suppressed. For high-precision measurement, it is necessary to calibrate the sensitivity under actual performance conditions, and this objective can be achieved by using a chip-level calibration coil to generate a magnetic field along the sensing direction of the sensor.
In addition, the magnetoresistive sensor is comprised of ferromagnetic sensing elements, and therefore, an output curve is non-linear. The generation of hysteresis is caused by the movement of domain walls within the sensing elements and other parts (e.g., a magnetic shielding layer or a flux concentrator layer). To overcome the above problems, a high-performance magnetoresistive sensor generally needs another coil to provide a periodic saturation field for the sensing elements in order to eliminate magnetic domains, and this coil is referred to as an initialization coil.
Patent No. 201310409446.5 publicized a single chip Z-axis linear magnetoresistive sensor, as shown in FIG. 1, for measuring an external magnetic field in a Z direction, i.e., a direction perpendicular to a substrate. The single chip Z-axis linear magnetoresistive sensor includes a substrate 1, a plurality of elongated soft ferromagnetic flux concentrators 2 located on the substrate 1 and having a length direction being a Y-axis direction and a width direction being an X-axis direction, and magnetoresistive sensing unit arrays 4 and 5 located on upper surfaces or lower surfaces of the soft ferromagnetic flux concentrators 2. The magnetoresistive sensing unit arrays are arranged into a push magnetoresistive unit string 4 and a pull magnetoresistive unit string 5 along the Y-axis direction, which are respectively located on two sides of a Y-axis center line 3 of the soft ferromagnetic flux concentrator 2, and have a same distance from the Y-axis center line. The push magnetoresistive unit string 4 and the pull magnetoresistive unit string 5 are electrically connected to form a push-pull bridge. A pinned layer direction and a magnetic field sensing direction of the magnetoresistive sensing unit are along the X-axis direction. When an external magnetic field is applied in the Z-axis direction, the soft ferromagnetic flux concentrators 2 distort the Z-direction magnetic field into two magnetic field components that have X-axis and −X-axis magnetic field components, that is oppositely oriented directions with identical amplitudes. The two sensing direction magnetic fields are applied to the push magnetoresistive string 4 and the pull magnetoresistive string 5, thereby forming a push-pull magnetoresistive sensor.
FIG. 2 is a cross-sectional diagram of the single chip Z-axis linear magnetoresistive sensor. It can be seen that, the push magnetoresistive sensing unit string 4 and the pull magnetoresistive sensing unit string 5 are located on the substrate 1. The soft ferromagnetic flux concentrators 2 are located above the push magnetoresistive sensing unit string 4 and the pull magnetoresistive sensing unit string 5. Moreover, the single chip Z-axis linear magnetoresistive sensor further includes an electrode 6; insulation layers 7 and 8 located between layers and configured to isolate the electrodes of the magnetoresistive sensing units and from the magnetoresistive sensing units 4, 5 and the soft ferromagnetic flux concentrators 2; and a passivation layer 9 configured to protect the whole device.
The magnetoresistive sensing unit strings 4 and 5 in the single chip Z-axis linear magnetoresistive sensor shown in FIG. 1 and FIG. 2 are TMR magnetoresistive sensing units, each including a free layer, a pinned layer, and a central barrier layer. An initial magnetization direction of the free layer is the Y-axis direction, and the magnetization direction of the pinned layer, that is, the magnetic field sensing direction, is the X-axis direction. The single chip Z-axis magnetoresistive sensor described above can effectively measure a Z-axis magnetic field component, but it has the following problems:
1) In a wafer test stage, a complex Z-direction external magnetic field generation system needs to be designed, including an electromagnetic coil and an electromagnetic coil power supply. Moreover, the electromagnetic coil system needs to move along with a probe platform, thereby increasing costs for measurement, and affecting the efficiency of measurement.
2) Application and positioning of a magnetic field of the electromagnetic coil system are imprecise, affecting the precision of measurement.
3) Magnetic domains exists in the soft ferromagnetic thin film of the free layer, and when an external magnetic field is applied, movement of the magnetic domains is irreversible. As a result, after the external magnetic field is removed, the ferromagnetic thin film of the free layer cannot return to its initial state. As a result, hysteresis is produced, making it hard to guarantee the repeatability of the sensor.